1. Field of the Invention
The present invention relates to a lane line estimating apparatus that sets a virtual line on a driving lane corresponding to a lane line based upon an external environment ahead captured by imaging unit mounted on a vehicle.
2. Description of the Related Art
There have conventionally been known driving support apparatuses making various controls during driving on an express highway. There is the one of the driving support apparatuses described above that makes a driving control in which a driving environment ahead of a vehicle is detected, lane lines defining the left side and the right side of a driving lane on which the vehicle runs are detected based upon the detected driving environment data, and the vehicle is allowed to automatically run along the lane lines. There is also one of the driving support apparatuses described above that makes a lane keeping control in which an alarm is issued to alert a driver, when the vehicle is predicted to be deviated from the recognized lane-line positions.
It is necessary to accurately estimate the positions of the lane lines in order to correctly drive the vehicle along the lane lines. As a unit for estimating the position of a lane line, there has been known a technique of detecting an edge of a white line from a brightness difference between a road surface and the white line based upon an image ahead of the vehicle imaged by a camera (hereinafter referred to as an “in-vehicle camera”) mounted on the vehicle, so as to determine the position of the lane line from the edge of the white line.
However, on an express highway, there may be not only the right and left lane lines that define a driving lane, but also a double white line including a lane line and a guide line (so-called dot line) formed inside the lane line on a curve. In the technique of detecting an edge of a white line from a brightness difference between the edge of the white line and a road surface, an inner guide line of a double white line is liable to be erroneously recognized as a lane line, since the brightness difference more clearly appears in the case of the white line at the inner side of the road than in the case of the white line at the outer side of the road. In addition, there may be also a case that a track is formed so as to be parallel to a lane line. When a puddle is formed in this track, a high-intensity light is received due to the reflection light from the water surface. Therefore, this track might be erroneously recognized as a lane line.
As a countermeasure, Japanese Patent No. 3733875 describes a technique in which, when a white line detected based upon an image captured by an in-vehicle camera is determined to be a pseudo white line such as a puddle on a track or a double white line, a road parameter excluding a lateral position of the vehicle with respect to the lane line is set so as to prevent the pseudo white line from being erroneously recognized as a white line.
Japanese Patent No. 3603836 describes a technique described below. In the technique, right and left white-line candidate points, which are paired, are detected based upon an image captured by an in-vehicle camera. Among the detected paired right and left white-line candidate points, pairs of candidate points having a same position in the vertical direction are detected. A straight-line approximation equation is obtained from the relationship between the horizontal distance and the vertical position of each of the pairs of the candidate points. True white-line candidate points are extracted from the respective pairs of the candidate points based upon the difference between a horizontal distance calculated from the straight-line approximation equation based upon the position of the candidate point pairs in the vertical direction and a horizontal distance of the candidate point pairs obtained from the image. Then, a road parameter is set based upon the true white-line candidate points, whereby the precision in detecting the true white-line candidate points is enhanced.
The technique described in Japanese Patent No. 3603836 focuses on the point in which lane lines are provided on the left side and on the right side so as to be parallel to each other, and when the right and left lane lines, which are captured by the camera, are subject to a straight-line approximation, the widths of the lanes (lane widths) become zero at disappearance points. Specifically, in this technique, in the case in which the disappearance points agree with each other in the straight-line approximation even when plural straight-line candidates are present on the right and left sides, the straight-line candidates can be determined to be a double white line.
In the technique described in Japanese Patent No. 3733875, when the pseudo white line is detected, the road parameter excluding the lateral position is set. Therefore, when a driver intends to drive the vehicle on an optional position, such as the center of the driving lane, the driver cannot set the driving line of the vehicle at the center of the driving lane, because there is no data involved with the lateral position. Accordingly, the driving control becomes unstable.
A tire of a running vehicle frequently passes on the inner guide line (dot line) of the double white line painted on the road surface of an express highway, so that the inner guide line are frequently faded in most cases. Therefore, the brightness difference between the edge of the inner guide line and the road surface is small, resulting in that it might be difficult to detect the inner guide line. In this case, the candidate points of the true white line at the outside and the candidate points of the inner guide line cannot clearly be identified in the vicinity of the disappearance point. Therefore, in the technique described in Japanese Patent No. 3603836, the detection precision of the true white line becomes poor.